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+// @flow strict
+import inspect from '../jsutils/inspect';
+import isAsyncIterable from '../jsutils/isAsyncIterable';
+import { addPath, pathToArray } from '../jsutils/Path';
+
+import { GraphQLError } from '../error/GraphQLError';
+import { locatedError } from '../error/locatedError';
+
+import type { DocumentNode } from '../language/ast';
+
+import type { ExecutionResult, ExecutionContext } from '../execution/execute';
+import { getArgumentValues } from '../execution/values';
+import {
+ assertValidExecutionArguments,
+ buildExecutionContext,
+ buildResolveInfo,
+ collectFields,
+ execute,
+ getFieldDef,
+} from '../execution/execute';
+
+import type { GraphQLSchema } from '../type/schema';
+import type { GraphQLFieldResolver } from '../type/definition';
+
+import { getOperationRootType } from '../utilities/getOperationRootType';
+
+import mapAsyncIterator from './mapAsyncIterator';
+
+export type SubscriptionArgs = {|
+ schema: GraphQLSchema,
+ document: DocumentNode,
+ rootValue?: mixed,
+ contextValue?: mixed,
+ variableValues?: ?{ +[variable: string]: mixed, ... },
+ operationName?: ?string,
+ fieldResolver?: ?GraphQLFieldResolver<any, any>,
+ subscribeFieldResolver?: ?GraphQLFieldResolver<any, any>,
+|};
+
+/**
+ * Implements the "Subscribe" algorithm described in the GraphQL specification.
+ *
+ * Returns a Promise which resolves to either an AsyncIterator (if successful)
+ * or an ExecutionResult (error). The promise will be rejected if the schema or
+ * other arguments to this function are invalid, or if the resolved event stream
+ * is not an async iterable.
+ *
+ * If the client-provided arguments to this function do not result in a
+ * compliant subscription, a GraphQL Response (ExecutionResult) with
+ * descriptive errors and no data will be returned.
+ *
+ * If the source stream could not be created due to faulty subscription
+ * resolver logic or underlying systems, the promise will resolve to a single
+ * ExecutionResult containing `errors` and no `data`.
+ *
+ * If the operation succeeded, the promise resolves to an AsyncIterator, which
+ * yields a stream of ExecutionResults representing the response stream.
+ *
+ * Accepts either an object with named arguments, or individual arguments.
+ */
+declare function subscribe(
+ SubscriptionArgs,
+ ..._: []
+): Promise<AsyncGenerator<ExecutionResult, void, void> | ExecutionResult>;
+/* eslint-disable no-redeclare */
+declare function subscribe(
+ schema: GraphQLSchema,
+ document: DocumentNode,
+ rootValue?: mixed,
+ contextValue?: mixed,
+ variableValues?: ?{ +[variable: string]: mixed, ... },
+ operationName?: ?string,
+ fieldResolver?: ?GraphQLFieldResolver<any, any>,
+ subscribeFieldResolver?: ?GraphQLFieldResolver<any, any>,
+): Promise<AsyncIterator<ExecutionResult> | ExecutionResult>;
+export function subscribe(
+ argsOrSchema,
+ document,
+ rootValue,
+ contextValue,
+ variableValues,
+ operationName,
+ fieldResolver,
+ subscribeFieldResolver,
+) {
+ /* eslint-enable no-redeclare */
+ // Extract arguments from object args if provided.
+ return arguments.length === 1
+ ? subscribeImpl(argsOrSchema)
+ : subscribeImpl({
+ schema: argsOrSchema,
+ document,
+ rootValue,
+ contextValue,
+ variableValues,
+ operationName,
+ fieldResolver,
+ subscribeFieldResolver,
+ });
+}
+
+/**
+ * This function checks if the error is a GraphQLError. If it is, report it as
+ * an ExecutionResult, containing only errors and no data. Otherwise treat the
+ * error as a system-class error and re-throw it.
+ */
+function reportGraphQLError(error: mixed): ExecutionResult {
+ if (error instanceof GraphQLError) {
+ return { errors: [error] };
+ }
+ throw error;
+}
+
+function subscribeImpl(
+ args: SubscriptionArgs,
+): Promise<AsyncGenerator<ExecutionResult, void, void> | ExecutionResult> {
+ const {
+ schema,
+ document,
+ rootValue,
+ contextValue,
+ variableValues,
+ operationName,
+ fieldResolver,
+ subscribeFieldResolver,
+ } = args;
+
+ const sourcePromise = createSourceEventStream(
+ schema,
+ document,
+ rootValue,
+ contextValue,
+ variableValues,
+ operationName,
+ subscribeFieldResolver,
+ );
+
+ // For each payload yielded from a subscription, map it over the normal
+ // GraphQL `execute` function, with `payload` as the rootValue.
+ // This implements the "MapSourceToResponseEvent" algorithm described in
+ // the GraphQL specification. The `execute` function provides the
+ // "ExecuteSubscriptionEvent" algorithm, as it is nearly identical to the
+ // "ExecuteQuery" algorithm, for which `execute` is also used.
+ const mapSourceToResponse = (payload) =>
+ execute({
+ schema,
+ document,
+ rootValue: payload,
+ contextValue,
+ variableValues,
+ operationName,
+ fieldResolver,
+ });
+
+ // Resolve the Source Stream, then map every source value to a
+ // ExecutionResult value as described above.
+ return sourcePromise.then((resultOrStream) =>
+ // Note: Flow can't refine isAsyncIterable, so explicit casts are used.
+ isAsyncIterable(resultOrStream)
+ ? mapAsyncIterator(
+ resultOrStream,
+ mapSourceToResponse,
+ reportGraphQLError,
+ )
+ : ((resultOrStream: any): ExecutionResult),
+ );
+}
+
+/**
+ * Implements the "CreateSourceEventStream" algorithm described in the
+ * GraphQL specification, resolving the subscription source event stream.
+ *
+ * Returns a Promise which resolves to either an AsyncIterable (if successful)
+ * or an ExecutionResult (error). The promise will be rejected if the schema or
+ * other arguments to this function are invalid, or if the resolved event stream
+ * is not an async iterable.
+ *
+ * If the client-provided arguments to this function do not result in a
+ * compliant subscription, a GraphQL Response (ExecutionResult) with
+ * descriptive errors and no data will be returned.
+ *
+ * If the the source stream could not be created due to faulty subscription
+ * resolver logic or underlying systems, the promise will resolve to a single
+ * ExecutionResult containing `errors` and no `data`.
+ *
+ * If the operation succeeded, the promise resolves to the AsyncIterable for the
+ * event stream returned by the resolver.
+ *
+ * A Source Event Stream represents a sequence of events, each of which triggers
+ * a GraphQL execution for that event.
+ *
+ * This may be useful when hosting the stateful subscription service in a
+ * different process or machine than the stateless GraphQL execution engine,
+ * or otherwise separating these two steps. For more on this, see the
+ * "Supporting Subscriptions at Scale" information in the GraphQL specification.
+ */
+export function createSourceEventStream(
+ schema: GraphQLSchema,
+ document: DocumentNode,
+ rootValue?: mixed,
+ contextValue?: mixed,
+ variableValues?: ?{ +[variable: string]: mixed, ... },
+ operationName?: ?string,
+ fieldResolver?: ?GraphQLFieldResolver<any, any>,
+): Promise<AsyncIterable<mixed> | ExecutionResult> {
+ // If arguments are missing or incorrectly typed, this is an internal
+ // developer mistake which should throw an early error.
+ assertValidExecutionArguments(schema, document, variableValues);
+
+ return new Promise((resolve) => {
+ // If a valid context cannot be created due to incorrect arguments,
+ // this will throw an error.
+ const exeContext = buildExecutionContext(
+ schema,
+ document,
+ rootValue,
+ contextValue,
+ variableValues,
+ operationName,
+ fieldResolver,
+ );
+
+ resolve(
+ // Return early errors if execution context failed.
+ Array.isArray(exeContext)
+ ? { errors: exeContext }
+ : executeSubscription(exeContext),
+ );
+ }).catch(reportGraphQLError);
+}
+
+function executeSubscription(
+ exeContext: ExecutionContext,
+): Promise<AsyncIterable<mixed>> {
+ const { schema, operation, variableValues, rootValue } = exeContext;
+ const type = getOperationRootType(schema, operation);
+ const fields = collectFields(
+ exeContext,
+ type,
+ operation.selectionSet,
+ Object.create(null),
+ Object.create(null),
+ );
+ const responseNames = Object.keys(fields);
+ const responseName = responseNames[0];
+ const fieldNodes = fields[responseName];
+ const fieldNode = fieldNodes[0];
+ const fieldName = fieldNode.name.value;
+ const fieldDef = getFieldDef(schema, type, fieldName);
+
+ if (!fieldDef) {
+ throw new GraphQLError(
+ `The subscription field "${fieldName}" is not defined.`,
+ fieldNodes,
+ );
+ }
+
+ const path = addPath(undefined, responseName, type.name);
+ const info = buildResolveInfo(exeContext, fieldDef, fieldNodes, type, path);
+
+ // Coerce to Promise for easier error handling and consistent return type.
+ return new Promise((resolveResult) => {
+ // Implements the "ResolveFieldEventStream" algorithm from GraphQL specification.
+ // It differs from "ResolveFieldValue" due to providing a different `resolveFn`.
+
+ // Build a JS object of arguments from the field.arguments AST, using the
+ // variables scope to fulfill any variable references.
+ const args = getArgumentValues(fieldDef, fieldNodes[0], variableValues);
+
+ // The resolve function's optional third argument is a context value that
+ // is provided to every resolve function within an execution. It is commonly
+ // used to represent an authenticated user, or request-specific caches.
+ const contextValue = exeContext.contextValue;
+
+ // Call the `subscribe()` resolver or the default resolver to produce an
+ // AsyncIterable yielding raw payloads.
+ const resolveFn = fieldDef.subscribe ?? exeContext.fieldResolver;
+ resolveResult(resolveFn(rootValue, args, contextValue, info));
+ }).then(
+ (eventStream) => {
+ if (eventStream instanceof Error) {
+ throw locatedError(eventStream, fieldNodes, pathToArray(path));
+ }
+
+ // Assert field returned an event stream, otherwise yield an error.
+ if (!isAsyncIterable(eventStream)) {
+ throw new Error(
+ 'Subscription field must return Async Iterable. ' +
+ `Received: ${inspect(eventStream)}.`,
+ );
+ }
+ return eventStream;
+ },
+ (error) => {
+ throw locatedError(error, fieldNodes, pathToArray(path));
+ },
+ );
+}